1. Field of the Invention
The present invention relates to optical fiber interferometric sensing systems and, more particularly, to an apparatus and method for the elimination of polarization induced fading in such systems.
2. Description of the Background
Stress, strain and fatigue often lead to failure in static and dynamic structural systems. Strain monitoring of such systems can provide an indication of the structural health of the system and predict failures, providing time to alter the system to potentially avoid the failure. Implementation of fiber-optics strain sensors in structural health monitoring is advantageous over traditional strain gauges due to the compactness of fiber optics sensors, the ability to cascade multiple fiber sensors onto a single fiber (as opposed to individual cabling for each strain gauge), and the inherent immunity of optical systems to electromagnetic interference. This technology has been implemented in both and static structures, such as bridges, roads and tunnels, and dynamic structures such as airframes and sea vessels, for real-time health monitoring. A particularly useful feature of fiber optic sensors is that they can provide distributed sensing spanning very large distances.
Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber. Interferometric fiber optic sensors measure the phase change of light traveling in an optical fiber due to the strains developed in the fiber by an applied pressure. Interferometric fiber optic sensors are extremely sensitive, allowing detection of fiber length variations on the order of a few hundred femtometers.
Several schemes are currently deployed to interrogate distributed optical fiber sensors: including Wavelength Division Multiplexing (WDM), Optical Time Domain Reflectometry (OTDR), and Optical Frequency Domain Reflectometry (OFDR).
WDM multiplexes a number of optical interrogation signals onto a single optical fiber by using different wavelengths (colors) of laser light. Each optical sensor is given a dedicated operating wavelength, and the wavelengths of the gratings are varied so as to be individually distinguished (each reflects a different wavelength of light from a broad-band or swept-wavelength laser source). A demultiplexer at the receiver splits the reflected signals apart for analysis. WDM achieves high spatial resolution but the number of possible sensors able to be deployed is limiting since the wavelength bandwidth is finite, and there is greater complexity in fiber manufacture.
OTDR is based on the premise of sending a pulse of laser light through the fiber and tracking the time at which the reflected signals are detected in order to distinguish the spatial location of each sensor. OTDR is effective for sensing over long distances but suffers from limited spatial resolution.
Some schemes combine OTDR and WDM to increase the number of addressable sensors but retain similar limitations.
With OFDR interrogation, a tunable laser wavelength scans a distributed optical system under test. The resulting internal reflections from the optical system interfere with the reflection from a reference reflector, producing a composite modulation of the interferometric signal, where the beat frequencies of the modulation are directly related to the position of the internal reflections. Taking the Fourier transform of the interferometric signal, it is possible to determine the magnitude and position of the internal reflections. OFDR has shown great promise to interrogate hundreds and even thousands of distributed optical fiber sensors.
These fiber optics strain sensors are typically composed of numerous optical fibers, and numerous Fiber Bragg gratings (“FBGs”) interposed along the length of each fiber. Each Bragg grating creates a periodic variation of the optical refractive index in the core of the optical fibers and is capable of detecting strain individually through change in its resonant wavelength (i.e., the wavelength at which each grating has its maximum reflectance).
With OFDR all the FBGs are written at the same central wavelength, and their positions along the fiber are detected by measuring the beat frequency of any individual grating's reflection with the reflection from a reference arm of the interferometer having a known length. As the laser wavelength (and therefore frequency) is scanned a small but measurable difference in the time for light to travel from the reference arm and from each grating is observed causing variation in the interference pattern between the signal from the reference arm and the sensor arm, the frequency components of which are linearly related to the distance along the fiber of the grating reflector. This wavelength or frequency domain interference pattern is presented as an “interferogram” (a graph of the variation of the output signal in the wavelength domain which encodes all beat frequencies of the sensor). A Fast Fourier Transform (FFT) signal-processing method is applied to convert the wavelength/frequency domain to the spatial/time domain so that the beating frequency for each FBG encoded within the interferogram can be separated. The Fourier transform of the reflected interferogram gives the signal reflectance as a function of time which, in this case, is equivalent to distance along the fiber. Thus the reflectivity of each grating can be measured. To transform the wavelength (and strain) information of a particular sensor from the spatial domain back to the wavelength domain, a windowing function is first taken to lock-in the desired beating frequency in spatial domain. Then, an inverse Fourier Transform is taken to identify the resonant wavelength of the particular sensor. Since strain information is linearly proportional to the wavelength shift relative to the initial resonant wavelength of the fiber grating, the strain information can be correlated from the wavelength shift information.
Other techniques to decode a particular sensor from the multitude of beat frequency information have been developed. However, regardless of the techniques used to determine the beat frequency, there is an inherent limitation of fiber interferometer sensing technology. Because an interferometric fiber-optic sensor combines the light beams from two optical paths and causes them to interfere to produce an interference pattern that is proportional to the property being sensed, the interference between the light beams from the two paths can fade due to polarization drifts. If the states of polarizations of the two interfering beams from the two arms of the interferometer are co-directional, the interference is at a maximum and the sensitivity of the sensor is greatest. However, if the states of polarizations of the beams from the two arms are orthogonal to each other, the light from the two arms do not interfere and the sensitivity of the sensor is zero. It is well known that the single mode fiber (SMF) used in fiber sensors is a highly bi-refringent optical waveguide and that physical perturbation (such as induced-strain, temperature, or rotation) between the arms of the interferometer can cause changes in the polarization of the waveguide. As a result, random fluctuations in the state of polarization (SOP) of the interfering beams guided in the nominally circular fiber core can lead to fading of the interference signal. This phenomenon is known commonly as polarization-induced fading (PIF).
An inherent solution to eliminate PIF is replacing the highly bi-refringent single-mode fiber with polarization-maintaining fiber (PMF). A PMF does not exhibit any polarization-related intensity change due to physical perturbation, and so the effect of PIF can be eliminated by utilizing PMFs throughout the fiber network. Unfortunately, this approach is not practical because PMFs are significantly more expensive than the SMF-based fiber networks, and so fiber strain sensors based on PMF has not been realized nor commercialized. Other techniques have been proposed to overcome the effect of polarization induced signal fading ranging from the use of polarization controllers in the arms of the fiber interferometer, to polarization input controllers, to polarization diversity detection schemes based on output polarization state selection. In the case of polarization controllers, active controllers such as those based on Faraday rotators or rotatable fiber wave plates can be inserted into one or both of the fiber arms. While this approach is well suited for use in laboratory sensor systems, it is not a practical solution for a deployable sensor and in any event is incompatible with a FBG based system as the gratings already act as reflectors. With respect to changing the source input polarization control, such systems involve additional components such as polarization state controllers to continuously alter the source polarization and may not be a practical solution for systems having thousands of sensors needed to match every single sensors polarization based on feedback from an output detector
In the case of polarization diversity detection, fading can be overcome by appropriately selecting a polarization mode at the output of the interferometer. Most polarization diversity detection schemes include three photo-detectors which are 60 degrees apart from one another in terms of their angle of polarization rotation. The advantage of this approach is to ensure that a completely faded signal caused by PIF will not occur. However, because PIF causes each fiber sensor in a multisensory network to experience randomly different degrees of polarization, using polarization diversity detection to selectively choose the best output of the network does not work. Consequently, a more effective apparatus and method for eliminating polarization-induced fading from all sections of the fiber sensor is needed.